1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to an ink ejecting method and an ink-jet printhead utilizing the method.
2. Description of the Related Art
Typically, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. Ink-jet printheads are largely categorized into two types depending on which ink droplet ejection mechanism is used. A first type is a thermally driven ink-jet printhead in which a heat source is employed to form and expand bubbles in ink causing ink droplets to be ejected. A second type is a piezoelectrically driven ink-jet printhead in which a piezolectric crystal bends to exert pressure on ink causing ink droplets to be ejected.
FIGS. 1A and 1B illustrate examples of a conventional thermally driven ink-jet printhead. FIG. 1A illustrates a cutaway perspective view of a structure of a conventional ink-jet printhead. FIG. 1B illustrates a cross-sectional view for explaining an ink droplet ejection mechanism of the conventional ink-jet printhead shown in FIG. 1A.
The conventional thermally driven ink-jet printhead shown in FIGS. 1A and 1B includes a manifold 22 provided on a substrate 10, an ink channel 24 and an ink chamber 26 defined by a barrier wall 14 installed on the substrate 10, a heater 12 installed in the ink chamber 26, and a nozzle 16 that is provided on a nozzle plate 18 and through which ink droplets 29′ are ejected. When a pulse-shaped current is supplied to the heater 12 and heat is generated in the heater 12, ink 29 filled in the ink chamber 26 is heated, and a bubble 28 is generated. The formed bubble 28 continuously expands and exerts pressure on the ink 29 contained within the ink chamber 26. This pressure causes the ink droplets 29′ to be expelled through the nozzle 16. Subsequently, ink 29 is absorbed from the manifold 22 into the ink chamber 26 through the ink channel 24, thereby refilling the ink chamber 26 with ink 29.
However, in the thermally driven ink-jet printhead, when ink droplets are ejected due to the expansion of bubbles, a portion of the ink in the ink chamber 26 flows backward to the manifold 22, and an ink refill operation is performed after ink is ejected. Thus, there is a limitation in implementing high printing speed.
Additionally, a variety of ink droplet ejection mechanisms as well as the two above-described ink droplet ejection mechanisms may be used in the ink-jet printhead and include an ink droplet ejection mechanism using an electrostatic force.
FIGS. 2A and 2B illustrate another example of a conventional ink droplet ejection mechanism and schematically show a principle of ink droplet ejection using an electrostatic force. FIG. 3 illustrates a schematic cross-sectional view of a conventional ink-jet printhead adopting the ink ejecting method shown in FIGS. 2A and 2B.
Referring to FIG. 2A, an opposite electrode 33 is disposed to be opposite to a base electrode 32, and ink 31 is supplied between the two electrodes 32 and 33. A DC power source 34 is connected to the two electrodes 32 and 33. When a voltage is applied from the power source 34 between the two electrodes 32 and 33, an electrostatic field is formed between the two electrodes 32 and 33. The electrostatic field causes a Coulomb force toward the opposite electrode 33 that acts on ink 31. At the same time, resistance against the Coulomb force acts on ink 31 due to the surface tension and viscosity of ink 31. Accordingly, ink 31 is not easily ejected to the opposite electrode 33. Thus, a very high voltage should be applied between the two electrodes 32 and 33 so that ink droplets are separated from the surface of ink 31 to be ejected. In this case, ejection of ink droplets occurs irregularly and a predetermined portion of ink 31 is heated locally. More specifically, temperature T1 of ink 31′ in a region S1 increases to be higher than temperature T0 of ink 31 in another region. Then, ink 31′ in the region S1 expands, and an electrostatic field is condensed on the region S1, and an electric charge is collected in the electrostatic field. As such, a repulsive force, acting between electric charges, and the Coulomb force, caused by the electrostatic field, act on ink 31′ in the region S1. Thus, as shown in FIG. 2B, ink droplets are separated from ink 31′ in the region S1 and move toward the opposite electrode 33.
Referring to FIG. 3, a pair of wall members 40 and 41 are spaced apart from each other, and ink 43 is filled therebetween. An exhaust hole 44 opposite to a recording paper 42 is provided on one side end of the wall members 40 and 41. A heating element 46 is installed at an inner side of the wall member 41, and electrodes 47 and 48 are connected to both ends of the heating element 46. A base electrode 49 for forming an electric field is provided at an inner side of the wall member 40. An opposite electrode 51 is installed at a rear side of the recording paper 42. A power source 52 for applying a voltage is connected to the opposite electrode 51, and the base electrode 49 is grounded. Another power source 53 is also connected to the both ends of the heating element 46. A control unit 54 for turning on/off the power sources 52 and 53 according to an image signal is connected to the power sources 52 and 53.
When a voltage is applied from the power source 52 between the base electrode 49 and the opposite electrode 51, ink 43 near the exhaust hole 44 is affected by the electric field. If a current is simultaneously applied from the power source 53 to the heating element 46, only ink 43 around the heating element 46 is ejected to the recording paper 42.
In the aforementioned conventional ink-jet printhead for ejecting ink using an electrostatic force, a very high voltage should be applied between two electrodes or ink should be locally heated by an additional heating element so that ink droplets are separated from the surface of ink to be ejected. These requirements increase power consumption. Due to electric charges irregularly collected on the surface of ink, it is very difficult to precisely control the volume and speed of ejected ink droplets. Thus, it is difficult to implement high-resolution printing.
Accordingly, in order to implement a low power consumption ink-jet printhead having high printing speed and high resolution, a new ink droplet ejection mechanism is needed.